1. Field of the Invention
The present invention relates to a plane array antenna for receiving satellite broadcasting programs to be utilized by being mounted on a car of the like.
2. Description of the Related Art
Along with the diffusion of satellite broadcasting in recent years, there have been many studies carried out on car-loaded antennas for receiving a satellite broadcast. For this type of car-loaded antenna, one of the important technical issues is how to reduce the height of the antenna technical because it usually is installed on the top of the car which operates under with height restrictions. Further, since the antenna for receiving a satellite broadcasting is fitted on the top of the car with a limited space, one of the important technical issues is also how to reduce the area for installing the antenna. To reduce the height for installation of the antenna, a structure for horizontally fitting the plane array antenna having a tilt angle on the top of the car is considered to be advantageous. Main beams irradiated from this type of plane array antenna are irradiated in a direction which is deviated by a tilt angle from the normal direction of the plane array antenna.
In a car-mounted antenna, an automatic tracking mechanism for controlling both an azimuth angle and an elevation angle of the antenna is necessary so that the antenna can always track a broadcasting satellite that continually changes with movement of the car. The automatic tracking mechanism not only constitutes a substantial portion of the manufacturing expense of the whole receiving system but also increases the height and area needed for the antenna. Therefore, simplification of the automatic tracking system has been one of the important technical issues. Changes of an azimuth angle occur over 360.degree. with movement of the car, and therefore, it is considered realistic to achieve the tracking in the direction of the azimuth angle by a mechanical rotation mechanism. In contrast to this trend, changes of an elevation angle occur along with longitude or the slope from the horizontal plane, that is, the slope of the load of about .+-.5.degree.. Therefore, the range of the changes of the elevation angle is relatively limited. As a result, it has been considered advantageous to economize the whole receiving system by employing an elevation angle direction non-tracking system not performing mechanical tracking in the elevation angle, or a uniaxial tracking system for tracing only the azimuth angle direction, by setting in advance the wider directivity in the elevation angle direction of the antenna.
A plane array antenna for receiving a satellite broadcasting which is designed to achieve the above-described uniaxial tracking system is described in the paper (A.P 93-25) titled "A SINGLE-LAYER STRUCTURE LEAKAGE WAVE GUIDE SLOT ARRAY CAR-LOADED ANTENNA FOR RECEIVING SATELLITE BROADCASTING", reported by Hirokawa et al. in the technical research report of the Institute of Electronics, Information and Communication Engineers (Japan), held in May 1993. This paper describes a leakage wave guide slot array antenna of a type, in which electric power is supplied in the rotation center (hereinafter, this type will be called as a central power supply type), having a structure as shown in the perspective view in FIG. 5. A main body of the slot array antenna is formed by 12 radiation wave guides 11A to 11L disposed mutually adjacent and parallel with each other, and T-shaped power supply wave guides 12 for supplying radiation power to each radiation wave guide. Each of the T-shaped power supply wave guides 12 is structured by a first part 12A which extends in its layout direction (or row direction) by forming a combining window with one end of each radiation wave guide and a second part 12B which extends from a power supply probe 13 formed at the rotation center position in the azimuth angle direction of the antenna main body, both the first and second parts 12A and 12B forming a T branch. Each of the radiation wave guides 11A to 11L is structured by a leakage wave guide which is formed with cross slots 14 in the axial direction by a suitable number, for example, 13 to 17, each having the same offset, to achieve a beam width of about .+-.5.degree. around the tilt angle direction of 52.degree..
The above paper suggests an advantage in that, in to the structure of the central power supply type shown in FIG. 5, when a power supply portion formed by the power supply probe 13 disposed at the center of rotation and is structured as a rotary joint or the like, only the antenna can be rotated within an almost horizontal plane at the time of uniaxial tracking while keeping the converter connected to this power supply portion at the lower side of the antenna main body in a fixed position.
The above paper by Hirokawa et al. shows a structure which enables only the antenna body to be rotated by employing the central power supply type antenna structure and the power supply portion having the rotary joint structure. However, further investigation is necessary in order to achieve an optimal structure. In the central power supply type antenna, the power supply system and the mechanical system for the rotation are complicated and concentrate at the center portion of the antenna because the center portion is important for both of the systems. An attempt to avoid the complexity of both the electrical and mechanical systems would-result in insufficient electrical and mechanical characteristics. If the power supply portion of the rotary joint structure and the converter are connected with a flexible coaxial cable, for example, it is possible to release the converter from the center portion and thus concentrate the rotation mechanical system in the center portion. However, since the frequency of the signal is as high as 12 GHz, a longer coaxial cable causes an increase in the transmitting loss and deterioration of the S/N ratio. On the other hand, fixing of the converter at the center portion of the antenna main body causes a problem that the converter becomes a hindrance so that a usual rotation mechanism combining the rotational axis of the motor with the center of rotation of the antenna main body can not be employed.